The present invention relates in general to processing image bearing signals read-out from an image sensing charge-transfer device (CTD) and, more particularly, to a method and apparatus which provides multiple read-out speeds for processing such image bearing signals. While the present invention is generally applicable to CTD's, it will be described herein with reference to an image sensing charge-coupled device (CCD) for which it is initially being used.
Image sensing charge-coupled devices (CCDs) are the most sensitive imaging devices currently available. CCDs also have a wide ratio between the strongest and weakest signals which they can capture, i.e., dynamic range, good sensitivity and can provide images at television rates. In view of these properties, CCDs are widely used in all areas of imaging technology.
In scientific imaging applications of CCDs, for example in a transmission electron microscope (TEM), to obtain the highest possible image quality, it is necessary to achieve the lowest possible signal noise and the widest possible dynamic range from an associated CCD. Reduced read-out noise and wide dynamic range can be obtained when CCD images are read out slowly and by means of a commonly used technique known as correlated double sampling. In essence, correlated double sampling subtracts a reset reference or "zero" voltage from an image voltage for each pixel of the CCD. Thus, for correlated double sampling, a new "zero" level is measured for the signal detected on each pixel of the CCD and subtracted from the signal level to substantially eliminate spurious contributions due to noise which is added to the signal level and the "zero" level in the same manner. Correlated double sampling is commonly performed using analog devices with the resulting correlated signal level then being digitized for further processing.
During operation of scientific imaging instruments, for example in TEMs during times when a specimen is being searched to identify an area which is to be investigated in greater detail, increasing the refresh rate of the image is more important than image quality to speed up the search process. In order to be able to obtain the best possible read-out noise performance at a given speed, all portions of the read-out circuitry need to be designed with a time constant matched to just the one given speed. Thus, with conventional correlated double sampling read-out circuitry, it is not possible to optimize the read-out operation for several different read-out speeds. In view of these limitations, currently available CCD imaging devices utilize circuitry which is either optimized for a single read-out speed or has more than one read-out speed serviced by multiple front ends of the read-out circuitry.
There is, thus, a need for an improved read-out arrangement which provides multiple read-out speeds for a CTD imaging device and optimizes the read-out operation for each available speed.